This invention relates to electromagnetic projectile launchers and more particularly to such launchers which include structures for recovering inductively stored rail energy or for accelerating current decay following the launch of a projectile.
In a simple parallel rail electromagnetic launcher system, the inductively stored rail energy at the time of projectile exit is equal to 1/2 L'.times.I.sup.2.sub.m where L' is the barrel inductance gradient, .times. is the barrel length, and I.sub.m is the muzzle current. The instantaneous accelerating force on a projectile is 1/2 L'I.sup.2 and, therefore, the total energy E imparted to a projectile, neglecting friction, is given by the equation: EQU E=.intg.Fdx=.intg.1/2 L'I.sup.2 dx (1)
If launcher barrel length is to be minimized, the maximum allowable force must be sustained throughout the bore length. In that case, the current must stay constant at its maximum level and, immediately following a projectile launch, the remaining rail inductive energy will be 1/2 L'.times.I.sup.2 which is then exactly equal to the projectile kinetic energy. Thus, for minimum barrel length and constant accelerating current, neglecting all rail resistance, 50% of the energy supplied at the breech will remain inductively stored by the rails and this energy is normally wasted. In practice, and expecially if high efficiency is desired, the projectile is allowed to exit only after a significant current drop compared to the initial breech current, but this involves the penalty or detriment of a considerably longer barrel.
If the barrel has external augmenting turns, then at least some of the post-projectile-exit inductively stored rail energy can be returned to the inductive storage in the power supply. Such a launching system is disclosed in a copending commonly assigned application by Kemeny et al. entitled "Electromagnetic Projectile Launcher With Energy Recovering Augmenting Field And Minimal External Field", filed Apr. 23, 1981, Ser. No. 256,745 (W.E. Case 49,429). The beneficial energy conservation illustrated in that application is available because of the augmenting turns and is unavailable for a simple parallel rail launcher. For a simple parallel rail launcher configuration, a muzzle shunt resistor can be used to dissipate inductively stored rail energy. The rate of current commutation into the muzzle resistor is improved by adding higher impedance sections to the projectile launching rails at the muzzle ends.
The shunting resistor can be designed such that it does not produce an excessive voltage drop but still dissipates most of the energy and thus results in less heating of the projectile launching rails. In general, the rate of current decay after firing will be substantially slower than the current decay during the actual projectile acceleration, and for very rapid burst firing, this relatively slow decay will limit the attainable burst firing rate. Current decay can be accelerated by increasing the resistance of the muzzle shunt impedance, which decreases the circuit time constant, but this results in higher voltage across the shunt and more difficulty in commutating current into the shunt. The present invention provides launching systems which improve the rate of current decay after projectile exit and can recover a portion of the inductively stored rail energy for use in accelerating a successive projectile.